The problem which is solved by the invention is how to bring down the final torque overshoot at so called stiff screw joints, an overshoot which is caused by the inertia of the rotating parts of the tool. At stiff joints the torque growth takes place very rapidly, because at these joints there is no deformation sequence during which the kinetic energy of the rotating parts of the tool is absorbed before the final torque level is reached. The overshoot problem is particularly significant at electrically powered tightening tools, because the rotating mass of an electric motor is much greater than that of a pneumatic vane motor.
In accordance with this invention the tightening process is carried out in two steps, namely a first, high speed step during which the screw joint is run down and tightened up to a snug level, and a second, low speed step during which the desired final torque level is reached. Since the first step ends at a snug level which is just a fraction of the final torque level the kinetic energy of the fast rotating motor and other rotating parts of the tool has practically no possibility at all to influence upon the final torque level, not even at extremely stiff joints. The result is a tightening tool with a very low mean shift, e.i. a tool which delivers torque with a very small or no difference at all between the installed torque at soft joints and the installed torque at stiff joints.
Two-step tightening is in itself an old and well known technique. For example, in U.S. Pat. No. 3,696,871 there is described a tightening tool having a two-speed gear mechanism which is shifted from a high speed/low torque operation to a low speed/high torque operation as a torque snug level is reached. This shifting of operation mode is accomplished by a torque responsive clutch which at a predetermined torque level disengages, whereby a planetary reduction gearing is engaged in the power transmission to reduce the speed and amplify the torque.
Apart from the fact that this known tool comprises not only a torque responsive release clutch but also a freewheel coupling and an extra reduction gearing, the snug level change of operation into the low speed/high torque mode takes place almost instantaneously, without any intermission in the power supply from the motor of the tool. This means that the inertia forces from the rotating parts "upstream" of the clutch are not at all reduced, but will add to the drive torque as the second tightening step commences. This is a serious disadvantage, because at stiff joints this might cause a substantial over-shoot at the desired final torque level.
Instead of employing a gear shift mechanism as described above, other previously known screw joint tightening systems operate in a two-step mode which is accomplished simply by interrupting the power supply to the tool and terminate the first step at a predetermined threshold torque level and after a certain time intermission restart the tool for a second tightening step. A tightening system of this type is disclosed in U.S. Pat. No. 3,965,778. Neither is a system of this type able to avoid torque overshoot at stiff joints. Due to the inertia forces of the rotating parts of the tool, and due to the fact that a high runningdown speed is always used during the first tightening step, the overshoot at the end of the first step sometimes extends beyond the desired final torque level. The reason is that the torque application on the joint is not discontinued fast enough.
The object of the present invention is to avoid the torque over-shoot problems inherent in the above described prior art.
It is also an object of the invention to provide a power tool for two-step screw joint tightening by which the above mentioned problems are solved and which is of a simple and compact design.